1. Field of the Invention
The present invention relates to a semiconductor device where an element such as a transistor is formed using a semiconductor film formed over a substrate. Moreover, the present invention relates to a laser irradiation method for annealing the semiconductor film for example. Furthermore, the present invention relates to a method for manufacturing a semiconductor device including the laser irradiation in its step.
2. Related Art
In recent years, a technique to form a thin film transistor (hereinafter referred to as a TFT) over a substrate has made great progress and application development to an active matrix display device has been advanced. Particularly, TFTs formed using poly-crystalline semiconductor films are superior in field-effect mobility to TFTs formed using conventional amorphous semiconductor films, and therefore high-speed operation becomes possible. For this reason, a circuit for driving pixels, which has been mounted on an external IC chip, can be formed integrally with the pixels over the same substrate using TFTs.
The poly-crystalline semiconductor film suitable for manufacturing a TFT is obtained by crystallizing an amorphous semiconductor film. To crystallize the amorphous semiconductor film, a laser annealing method is generally employed. The laser annealing is more preferable than a general thermal annealing that requires a temperature as high as 600° C. or more. This is because an inexpensive glass substrate is inferior in heat resistance and is easy to change in shape due to the heat. That is to say, the laser annealing has advantages that the processing time can be shortened to a large degree compared with another annealing method using radiation heat or conduction heat, and that a semiconductor substrate or a semiconductor film on a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally. Therefore, the laser annealing method is widely used to crystallize the amorphous semiconductor film formed over the glass substrate.
It is noted that the laser annealing method described herein includes the technique to recrystallize an amorphous layer or a damaged layer formed in the semiconductor substrate or the semiconductor film, and the technique to crystallize an amorphous semiconductor film formed over the substrate. In addition, the technique to planarize, modify the surface of the semiconductor substrate and the semiconductor film are also included.
Laser oscillators are classified broadly into two kinds, which are a pulsed laser oscillator and a continuous wave laser oscillator, in a standpoint of the oscillation method. In recent years, it has been known that the continuous wave (hereinafter referred to as CW for short) laser oscillator such as an Ar laser and a YVO4 laser can enlarge a crystal grain size formed in the semiconductor film compared with the pulsed laser oscillator such as an excimer laser. The CW laser beam used in the laser annealing has a wavelength in the visible region that is sufficiently absorbed in the amorphous semiconductor film, and that is easily obtained from the laser oscillator. When the crystal grain in the semiconductor film becomes larger, the number of grain boundaries in a channel-forming region of a TFT formed using the semiconductor film decreases. Therefore, the mobility becomes so high that this semiconductor film can be used to develop a more sophisticated device. For this reason, attention has been paid to the technique for crystallizing the semiconductor film using the CW laser oscillator.
However, when the laser annealing is performed using the CW laser oscillator, there is a problem that the annealing is not performed uniformly to a semiconductor film. This is because the laser beam emitted from the laser oscillator has Gaussian intensity distribution where the intensity decreases from the center toward the edge. Thus, it is difficult to anneal the semiconductor film uniformly.